The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 100 22 087.8, filed on May 8, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention concerns a pressing section for pressing water out of a paper web. Pressing sections have become known in numerous configurations. The invention relates to such pressing sections in which at least two pressing stations are present, of which at least the second pressing station is formed of a so-called shoe press and in which the paper web is pressed between two felts.
2. Discussion of Background Information
Such pressing sections have already been used for a long time in the production of thicker packaging papers. Such papers are produced at medium speeds. Here, the above-mentioned pressing sections have proven themselves. See Wochenblatt fxc3xcr Papierfabrikation [Paper Manufacturing Weekly], No. 6, 1999; p. 380-385, in particular FIG. 19. It is possible to use relatively coarse felts for these packaging papers that do mark the paper web, but have a high air permeability.
More recently, the attempt has been made to use pressing sections of the above-mentioned type in a slightly modified construction for the production of thinner papers, which are produced at higher speeds, for example, printing papers. They are described in xe2x80x9cWochenblatt fxc3xcr Papierfabrikationxe2x80x9d [xe2x80x9cPaper Manufacturing Weeklyxe2x80x9d], No. 2, 1999; p. 94 to 99, FIGS. 7, 11, and 16 as well as in Wochenblatt fxc3xcr Papierfabrikation [Paper Manufacturing Weekly], No. 11/12, 1999; p. 745 to 752, figure of the PM 5 Compact Lang Papier, Ettringen on page 746. It has been shown that pressing sections of the type mentioned above are advantageous because both sides of the paper web have printing properties that are approximately identical in quality. However, the results have not been completely satisfactory up to now. Specifically, the problems are as follows:
Insufficient dry contents were achieved with pressing sections of the type mentioned above. Thus, the paper web is still relatively moist after leaving the pressing section. Furthermore, the dry content of the paper web is not even across the web width.
The separation of the paper web from one or the other felt after passing through the individual pressing stations is critical; occasionally, tearing of the edges of the paper web occurs. Besides the higher speeds, these problems are also connected to the fact that the felts for thin papers must be denser and smoother due to the quality requirements of the paper surfaces to be printed. Thus, their dewatering becomes more difficult. The function of the suction felt guide rolls becomes less certain. They are supposed to hold the paper web safely on the felt provided for further transport during the separation of the two felts. However, this latter is only successful when the transport felt is sufficiently open and air permeable.
The present invention provides a pressing section that includes a first and a second pressing station, a common lower felt arranged to run through the first and the second pressing stations, and an first upper felt and a second upper felt arranged to run through the first and second pressing stations. The second pressing station includes a shoe press roll located above a lower roll, and a pressing plane of the second pressing station is oriented one of essentially vertically or inclined at an angle xcex2, which is a maximum of about 20xc2x0 to a vertical reference. Moreover, according to the instant invention, the pressing section is more efficient to use for thinner papers at higher machine speeds as well, in particular for printing papers. Thus, such a pressing section should have its own advantages with respect to the even printability properties on both sides of the web. Moreover, it should be free from the above-mentioned disadvantages and should by all means allow a longer felt service life. It should also prevent the problem of tears in the edges of the paper web and reduce the number of suction felt rolls, their energy use, and their noise level.
Accordingly, the present invention also includes a common lower felt that is sufficiently dense to be essentially air impermeable even in its new state after moistening, and that is sufficiently open to allow water through at pressures greater than about 5 bar. The first and second upper felts are sufficiently open to allow air through in their moistened, uncompressed state, and a pressing plane of the first pressing station being oriented at an angle xcex1, which is greater than angle xcex2, and preferably where angle xcex1 is greater than about 20xc2x0 to the vertical reference.
Specifically, the inventor recognized the following:
For the purpose of better printability, the felts must have as smooth a surface as possible. Further, the water tightness of the felts is an extremely critical parameter. It must be set in such a way that a rewetting of the paper by the belts cannot occur after each pressing station. The lower felt must be made sufficiently watertight that it is not only air impermeable, but also, in the case of small differences in pressure and by capillarity, returns as good as no water to the paper web during a contact period of approximately 0.1 to 0.2 seconds after each of the two pressing sections.
Moreover, the upper felts must be open and air permeable so that the paper web remains securely adhered to the lower felt (to which it adheres due to its lack of air permeability) when the felts are being separated after the pressing stations and, as a result of the air flowing in from above, separates itself from the upper felt. Here, the contact time between the paper web and the upper felt after each pressing station should be minimized so that the reverse moistening of the paper web by the upper felt remains low.
The combination of features according to the invention provides an elegant selection from a plurality of features that leads to success. Specifically, the following advantages can be attained:
The web leaves the pressing section with a greater dry content than before, which leads to fewer tears in the paper web. This increases the production capacity. Further, there are no strips of different degrees of dryness (moist strips) in the web cross profile, and the felts need to be dewatered to a lesser degree. The necessary energy for providing the vacuum as well as the number of suction rolls can be reduced while increasing the service life of the felts, since the latter can be more specialized for one single purpose. Moreover, the number of felts itself is reduced such that the machine is also more accessible, especially in the cellar of the machine, and the noise level of the machine and the energy consumption of the press section during operation are reduced.
The following is specifically achieved in accordance with features of the instant invention:
The common lower felt being sufficiently dense to be essentially air impermeable even in its new state after moistening, and the first and second upper felts being sufficiently open to allow air through in their moistened, uncompressed state allow an essentially flawless separation of the web from the upper felt upon exiting the pressing station; the web reliably follows the lower felt, which is desired. The common lower felt being sufficiently open to allow water through at pressures greater than about 5 bar ensures the evenness of dewatering of the paper web towards both of its surfaces, which is essential for printability. A pressing plane of the first pressing station which is oriented at an angle xcex1, which is greater than angle xcex2, and preferably which is greater than about 20xc2x0 to a vertical reference contributes to the prevention of reverse moistening of the paper web by the upper felt upon exiting the pressing station.
In further embodiments, the jacket of the first upper pressing roll is grooved or blind bored. Thus, a storage capacity is created so that not as much emphasis is placed on the storage capacity of the upper felt of the first press. This upper felt is thus relieved in a certain manner. The result is that it can be optimized with respect to other requirements. It can now be constructed in such a way that it leaves behind less pronounced markings on the paper. Furthermore, it can be constructed to be lighter and more supple. This allows a faster and less expensive installation when changing felts.
If something is true for the upper felt, it is true in a similar manner for the lower felt, which is common to both presses, as well as for the upper felt of the second press. Each felt may be specialized for a particular task.
An interesting further embodiment of the invention includes of constructing the lower felt out of at least two layers. Here, the outer layer, which is in contact with the paper, has relatively little or hardly any air permeability and thus is relatively dense, smooth, and not very compressible. The inner layer, on the other hand, is very permeable and has a high water storage capacity. This variant especially comes into consideration when no use is made of the blind boring and/or grooving of the upper press roll.
With the above-mentioned two-layer structure, it is possible to discharge the water stored in the permeable inner layer with little expense and to remove it laterally from the felt loop.
The present invention is directed to a pressing section of a paper machine. The pressing section includes a first and a second pressing station, a common lower felt arranged to run through the first and the second pressing stations, and an first upper felt and a second upper felt arranged to run through the first and second pressing stations. The second pressing station includes a shoe press roll located above a lower roll, and a pressing plane of the second pressing station is oriented one of essentially vertically or inclined at an angle xcex2, which is a maximum of about 20xc2x0 to a vertical reference. The common lower felt is sufficiently dense to be essentially air impermeable even in its new state after moistening, and is sufficiently open to allow water through at pressures greater than about 5 bar. The first and second upper felts are sufficiently open to allow air through in their moistened, uncompressed state, and a pressing plane of the first pressing station being oriented at an angle xcex1, which is greater than angle xcex2.
In accordance with a feature of the instant invention, angle xcex1 can be greater than about 20xc2x0 to the vertical reference.
Further, the first pressing station may include an upper pressing roll having a jacket surface which is one of grooved or blind bored. The shoe press roll of the second pressing station may include a jacket surface which is one of grooved or blind bored.
The common lower felt may have at least two layers including an outer layer, arranged to contact a paper web, which is air impermeable in a moistened state and an inner layer which is permeable and capable of storing water. Water removal from the common lower felt may occur substantially exclusively inwardly. At least one dewatering station can be assigned to at least one of the common lower felt and the first and second upper felts. The at least one dewatering station can include a water receiving tank arranged behind a felt deflection roll. The water receiving tank may be formed generally around an outside of the felt deflection roll. The at least one dewatering station can be formed as a flushing device that includes a device for conducting flushing air to an inner side of the least one felt as well as suction slits for removing water stored in the at least one felt.
According to another feature of the invention, a suction roll arranged in a loop of the first upper felt of the first pressing station.
The first pressing station can include a shoe press roll arranged as a lower roll. Subsequent to a concave pressing surface, a press shoe of the lower roll of the first pressing station can have a convexly bent sector with a radius of about 40 to 80 mm and an extension of about 2.5 to 5xc2x0, upon which a sector with a greater radius follows.
An edge depressor may be arranged after at least one of the first and the second pressing stations. The first and the second upper felt have felt edges, and at least one of the felt edges of at least one of the first and the second upper felts can be deflected downwardly by the edge depressor after running through a respective pressing station. The deflection can occur over an edge region with a width of over about 100 mm, and the deflection may increase outwardly toward the felt edge. Further, the deflection may increase towards the felt edge from about 20 mm inside the felt edge.
The present invention is directed to a pressing section of a paper machine. The pressing section includes a first and a second press, in which the first and second presses are double felted presses. A lower felt is arranged to run through the first and the second presses, and the common lower felt is sufficiently dense to be essentially air impermeable even in its new state after moistening, and is sufficiently open to allow water through at pressures greater than about 5 bar. First and second upper felts, which are arranged to run through the first and the second presses, respectively, are sufficiently open to allow air through in their moistened, uncompressed state. A pressing plane of the first press is oriented obliquely with respect to a pressing plane of the second press.
According to another feature of the invention, an edge depressor can be arranged after at least one of the first and the second presses to deflect an edge of the at least one of the first and the second upper felts toward the common lower felt.
In accordance with yet another feature of the present invention, the first and second presses can include shoe presses, and a lower roll of the first press and an upper roll of the second press may include a shoe press roll.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.